This invention relates to a scanning electron microscope in general and more particularly to a scanning electron microscope with an electron-optical column for the contactless testing of a sample, preferably an electronic component, in particular an integrated circuit disposed in a sample chamber on a sample holder, an image of which sample is then reproduced on a viewing screen in larger scale.
As is known, contactless potential measurements can be made with a scanning electron microscope for the functional testing of electronic components, in particular integrated circuits. The scanning electron microscope consists essentially of an electron-optical column which contains an electron gun and is generally provided with equipment for gating the electron beam and for deflecting the electron beam. These devices are disposed within the electron column in a vacuum. The component to be tested is in a sample chamber which is also evacuated.
At the measuring point the primary electron beam releases secondary electrons from a conductor of the module which are collected by an electron collector and converted into electrical signals. The number of secondary electrons released by the primary electron beam at the measuring point depends on the potential at the measuring point. At positive potential, only relatively few secondary electrons are released from the conductor surface, and a correspondingly dim picture spot appears on a viewing screen. Zero potential and negative potential result in a greater number of secondary electrons with a correspondingly bright picture spot. Therefore, conductors of positive potential appear dark on the viewing screen, and conductors of negative potential result in bright portions of the reproduced picture.
Accordingly, the characteristics of the conductor potential can be determined by this potential contrast measurement and displayed on the viewing screen by scanning the conductors of the unit under test.
For quantitative potential measurements, the scanning electron microscope is provided with a spectrometer, such as a retarding field spectrometer. A cylindrical deflection capacitor leads the secondary electrons through a retarding field to the electron collector which is followed by a control amplifier whose output signal controls the spectrometer via a feedback path. The grid voltage at the retarding field electrode is readjusted until the voltage between grid and measuring point has regained its original, constant value. Then the grid voltage change corresponds directly to the potential change at the measuring point of the test sample.
For functional testing or defect analysis and for displaying the potential distribution through this potential measurement, integrated circuits, for example, including their ceramic casing are placed into the sample or specimen chamber. The electrical leads are combined in special cables and brought through the sample chamber wall by means of vacuum leadthroughs. However, long control lines are sensitive to interference, in particular when transmitting high-frequency control signals, and may lead to a flattening-out of the rising and falling edges and to the overshooting and undershooting of the signals as well as to cross-talk. The moving device required to position the sample relative to the primary electron beam is likewise in a vacuum and, therefore, must be driven externally by vacuumtight lead-throughs (Scanning Electron Microscopy/IITRI, Chicago (USA), April 1976 (Part IV), pages 615 to 624).
The scanning electron microscope is shut off and the sample chamber opened in order to exchange specimens. The air entering also flows into the electronic optical column, wherefore the danger of contaminating its sensitive components cannot be precluded. After replacing the test sample, both the sample chamber and the electron-optical column must be evacuated again.